In recent years, there has been an increasing trend towards the replacement of conventional CRT raster-scanning displays by new types of display device, such as liquid crystal display panels, which offer the advantages of compactness, flatness, light weight, low power consumption, etc. One of the features which differentiates such new types of display device from CRT types of display lies in the manner in which drive signals are applied to produce scanning of the display, since the new display devices are driven by purely digital timing signals, rather than the analog form of drive signal applied to drive a CRT display (e.g. to produce scanning in the vertical direction). There is therefore a requirement to implement all of the circuits associated with such a flat-panel type of display device by utilizing only circuit elements producing digital signals, for example in order to enable all or most of the peripheral circuits to be formed within a single IC chip. This has the advantage of enabling power consumption to be minimized as well as reducing manufacturing costs and enabling overall size reduction.
It is possible to utilize circuits for deriving the horizontal and vertical synchronization signals (referred to in the following as horizontal and vertical sync signals) for a driving a liquid crystal display panel which are similar to those employed hitherto for CRT television displays. These sync signal generating circuits are of basically analog type, and require the use of components (e.g. relatively large-value capacitor used in an integrator circuit for separating the horizontal and vertical sync signal components) which cannot be formed within an IC chip, and so must be provided external to the IC. In addition, such circuits may consume a substantially high level of power.
With prior art sync signal generating circuits which have been of digital type, for use in television receivers, logical processing is generally performed on the output signal from an oscillator at a frequency of approximately 15.75 KHz (e.g. whose period is equal to the horizontal scanning period of a television display, designated in the following as H). Such digital types of circuit have also been used for CRT display scanning. However since a very stable frequency of oscillation is required if the signals are to be used for horizontal scanning of a CRT display, an AFC circuit will generally be incorporated to control that frequency. Thus. the circuit configuration becomes complex.
ln addition, if interlace scanning is to be performed (i.e. for a CRT display in accordance with NTSC standards), it will be necessary to provide a different sync signal generating circuit from that used with a non-interlace scanning display, such as that of a computer. That is, in the prior art, it has not easy possible to provide a digital type of sync signal generating circuit which will be compatible with both interlace and non-interlace scanning.
There is therefore a requirement for a synchronization signal generating circuit which is of essentially digital type, i.e. can be formed substantially entirely of digital circuit elements which can be formed within a common IC chip, which will have low power consumption, will be suitable for scanning of both CRT and flat-panel (e.g. liquid crystal display panel) displays, and preferably will be suitable for both interlace and non-interlace scanning applications.